Conventionally, a time-slot method is used in the communication between a contactless IC card that receives data using an electromagnetic induction method and the reader/writer that identifies that contactless IC card. This is done in order to solve the problem of some contactless IC cards not being able to perform communication with the reader/writer when there is a plurality of contactless IC cards in the communication area of the reader/writer at the same time, and when this plurality of contactless IC cards send response signals in response to polling from the reader/writer and these response signals collide with each other.
The time-slot communication method will be explained below.
(1) In order to check the existence of a contactless IC card, the reader/writer sends an initial-response request to the contactless IC card. The initial-response request contains the number of time slots in which the contactless IC card can make an initial response, or in other words the ‘Number of Slots’.
(2) After the contactless IC card receives the initial-response request, it returns an initial response in a time slot that starts from a specified time (1 to ‘Number of Slots’: hereafter, called the slot). The response slot is set by the contactless IC card itself.
(3) When the reader/writer detects a collision of the initial response from the contactless IC card, it sends the initial-response request to the contactless IC card again to restart the ID process of the time-slot method for the contactless IC card.
(4) When the reader/writer does not detect any collisions of initial responses from the contactless IC cards in any of the slots, it is then possible to identify all of the contactless IC cards, and it completes the ID process sequence.
Also, there is a slot-marker method in which the reader/writer sends a notification of the response timing. The slot-marker method is a method in which the reader/writer sends a slot-marker command to indicate the start of the slot in start timing of each slot after the reader/writer sends the initial-response request in the time-slot method. The method in which the IC card uses random numbers to set the response slot is the same.
A wireless ID apparatus has been disclosed as this kind of system (Japanese unexamined patent publication No. 9-6934: document 1).
Also, a contactless IC card ID method has been disclosed in the contactless IC card ID system and ID method of Japanese unexamined patent publication No. 11-205334 (document 2).
However, in the wireless ID apparatus (in document 1), the slots for sending response signals are set by random numbers and do not depend on the contactless IC cards. Therefore, when a plurality of contactless IC cards generate the same random number, there will be collisions of initial responses. In that case, it is necessary for the reader/writer to send the initial-response request command again and start again the process for setting the time slots, so there is a delay for the reader/writer to identify the IC cards.
Also, in the contactless IC card ID system and ID method (in document 2), a response is made to the first request after the power has been turned ON using slot number 1, or in other words, the first time slot, and from the second request on, the slot numbers for the responses are set using random numbers as in the case of the wireless ID apparatus (in document 1).
In this case as well, when identifying a plurality of contactless IC card, there is always a collision in the first request. Therefore, similarly, the reader/writer must send the initial-response request command again and start again the process for setting the time slots, so there is a delay for the reader/writer to identify the IC cards.